Analog-to-digital conversion apparatus and method capable of achieving fast settling

ABSTRACT

A method utilized in an analog-to-digital conversion apparatus, for converting an analog input signal into a digital output signal including a first portion and a second portion, includes: using a comparator circuit to compare the analog input signal with at least one first reference level to generate a preliminary comparison result, the at least one first reference level being used for determining the first portion; estimating the first portion according to the preliminary comparison result; based on the preliminary comparison result, performing the successive approximation procedure to obtain a posterior comparison result according to a plurality of second reference levels, the second reference levels being used for determining the second portion; and, estimating the second portion according to the posterior comparison result. The preliminary and posterior comparison results are generated by the comparator circuit.

BACKGROUND

In general, a conventional analog-to-digital converter with successiveapproximation register operation (SAR ADC) is attractive for medium tohigh bit resolution due to an excellent power efficiency. However, theconventional SAR ADC requires several clock cycle to complete one bitconversion and is also limited to low-bandwidth application. The advanceof MOS technology and a 2-b/step design extends the conversion rate tohundreds of MHz. Unfortunately, the need of multiple capacitivedigital-to-analog converters (DAC) for the 2-bit SAR ADC results inlarge input loading. Due to this, the application for higher resolutionis constrained. A conventional resistive DAC-based design may beemployed to try to alleviate this problem. Unfortunately, a large numberof switches and complex routing of the resistive DAC limits itsreferences settling time and conversion rate.

SUMMARY

Therefore, one of the objectives of the present invention is to providean analog-to-digital conversion apparatus and a method utilized in theanalog-to-digital conversion apparatus, to solve the above-mentionedproblems.

According to an embodiment of the present invention, a method utilizedin an analog-to-digital conversion apparatus, for converting an analoginput signal into a digital output signal, is disclosed. The digitaloutput signal includes a most significant bit portion and a leastsignificant bit portion. The method comprising: using a comparatorcircuit to compare the analog input signal with at least one firstreference level to generate a preliminary comparison result withoutusing a successive approximation procedure, the at least one firstreference level being used for determining the most significant bitportion; estimating the most significant bit portion according to thepreliminary comparison result; based on the preliminary comparisonresult, performing the successive approximation procedure to obtain aposterior comparison result according to a plurality of second referencelevels, the second reference levels being used for determining the leastsignificant bit portion; and, estimating the least significant bitportion according to the posterior comparison result.

According to the embodiment of the present invention, ananalog-to-digital conversion apparatus for converting an analog inputsignal into a digital output signal is disclosed. The digital outputsignal includes a most significant bit portion and a least significantbit portion. The analog-to-digital conversion apparatus comprises afirst digital-to-analog converter (DAC), a second DAC, a comparatorcircuit, and a decoder. The first DAC is used for providing at least onefirst reference level for the comparator circuit, and the at least onefirst reference level is used for determining the most significant bitportion. The second DAC is used for providing a plurality of secondreference levels for the comparator circuit, and the second referencelevels is used for determining the least significant bit portion. Thecomparator circuit is coupled to the first DAC and the second DAC, andused for comparing the analog input signal with the at least one firstreference level to generate a preliminary comparison result withoutusing a successive approximation procedure. The decoder is coupled tothe comparator circuit and used for estimating the most significant bitportion according to the preliminary comparison result. Based on thepreliminary comparison result, the decoder is used with the comparatorcircuit to perform the successive approximation procedure to obtain aposterior comparison result according to the second reference levels,and the decoder is arranged to estimate the least significant bitportion according to the posterior comparison result.

According to an embodiment of the present invention, ananalog-to-digital conversion (ADC) apparatus for converting an analoginput signal into a digital output signal is disclosed. The digitaloutput signal includes a first portion and a second portion. Theanalog-to-digital conversion apparatus comprises a firstdigital-to-analog converter (DAC), a second DAC, a comparator circuit,and a decoder. The first DAC is used for providing at least one firstreference level being used for determining the first portion. The secondDAC is used for providing at least one second reference level being usedfor determining the second portion. The comparator circuit is coupled tothe first DAC and the second DAC, and used for comparing the analoginput signal with the at least one first reference level to generate apreliminary comparison result at an output of the comparator circuit,and for generating a posterior comparison result at the output accordingto the analog input signal and the at least one second reference level.The decoder is coupled to the comparator circuit, and used forestimating the first portion according to the preliminary comparisonresult, and for estimating the second portion according to the posteriorcomparison result.

Based on the embodiment of the present invention, fast settling of thereference level (s) for the comparison of the most significant bitportion is obtained. Further, by using the capacitive interpolation, anumber of NMOS transistor switches can be reduced, and a matchingrequirement of corresponding circuitry elements can be lowered down. Theneed of specific duty cycle of an external clock for defining samplingperiod in a conventional asynchronous successive approximationanalog-to-digital converter can be avoided. In addition, according tothe above-mentioned design, the analog-to-digital conversion apparatusconsumes less power and occupies a smaller area of a chip die. Also, alarge number of switches and a complicated routing are reduced greatly.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an analog-to-digital conversion (ADC) apparatusaccording to an embodiment of the present invention.

FIG. 2 is a diagram of a comparator block as shown in FIG. 1.

FIG. 3A in conjunction with FIG. 3B are a diagram of the detailcircuitry elements of capacitors and comparator blocks according to anembodiment of the present invention and a diagram of the proposed ADCtiming sequence according to this embodiment of the present invention.

DETAILED DESCRIPTION

Please refer to FIG. 1, which is a diagram of an analog-to-digitalconversion (ADC) apparatus 100 according to an embodiment of the presentinvention. As shown in FIG. 1, the analog-to-digital conversion (ADC)apparatus 100 is used for converting an analog input signal V_(in) intoa digital output signal V_(out), and it comprises a firstdigital-to-analog converter (DAC) 105, a second DAC 110, a comparatorcircuit 115, and a decoder 120. The first DAC 105 is arranged to provideat least one first level V₁ for the comparator circuit 115 based on atleast one first reference level (i.e. input level(s) of the first DAC105). The at least one first reference level is utilized fordetermining/estimating a first portion of a digital codeword of thedigital output signal V_(out), such as a most significant bit (MSB)portion. In this embodiment, the first level(s) V₁ can be equivalent tothe first reference level(s). The second DAC 110 is arranged to sampleand store the analog input signal V_(in) and provide a plurality ofsecond levels V₂ that are generated by adding second reference levels tothe sampled analog input signal V_(in) for the comparator circuit 115.The second reference levels (i.e. the inputs of the second DAC 110) areutilized for determining/estimating a second portion of the digitalcodeword of the digital output signal V_(out), such as a leastsignificant bit (LSB) portion. Examples in following paragraphs use thefirst DAC 105 to generate the MSB portion and the second DAC 110 togenerate the LSB portion. However, with slight modifications, the firstDAC 105 may be used to generate other portion, such as a centersignificant bit (CSB) portion or a LSB portion of the digital codewordof the digital output signal V_(out), and the second DAC 110 may be usedto generate a MSB portion or a CSB portion. In this embodiment, thedigital codeword of the digital output signal V_(out) is formed by theMSB portion and the LSB portion without other bits. However, this is notmeant to be a limitation of the present invention. In other embodiments,the digital codeword may be formed by other bit(s) such as a CSBportion.

The comparator circuit 115 is arranged to receive the at least one firstlevel V₁ before receiving the second levels V₂. The operation ofestimating the MSB portion is performed before estimating the LSBportion. At first, the analog input signal V_(in) is sampled and storedat the capacitors in the second DAC 110, and the decoder 120 is arrangedto enable the first DAC 105 (e.g. to turn on the selected switch(s) ofthe first DAC 105) and disable the second DAC 110 (e.g. to turn off theswitches of the second DAC 110), so that the comparator circuit 115receives the at least one first level V₁ generated by the first DAC 105and the analog input signal V_(in). When receiving the at least onefirst level V₁, the comparator circuit 115 is arranged to compare theanalog input signal V_(in) with the at least one first level V₁ (whichis the same as the first reference level in this embodiment) to generatea preliminary comparison result, and the decoder 120 estimates the MSBportion according to the preliminary comparison result. It should benoted that the operation of generating the preliminary comparison resultis performed without using a successive approximation procedure.

After estimating the MSB portion by the comparator circuit 115, thedecoder 120 is used with the comparator circuit 115 to perform asuccessive approximation procedure to obtain a posterior comparisonresult according to the above-mentioned second levels V₂ that aregenerated from the second DAC 110. The decoder 120 is then arranged todetermine the codeword of the digital output signal V_(out) byestimating the LSB portion according to the estimated MSB portion. Indetail, the decoder 120 is arranged to enable the second DAC 110 andcontrol on/off states of transistor switches in the second DAC 110 so asto provide appropriate second levels V₂, and initiate and execute thesuccessive approximation procedure to estimate the LSB portion. Thesecond levels V₂ are resultant levels generated from adding the analoginput signal V_(in) to the input voltages of the second DAC 110. Inother words, the comparator circuit 115 is arranged to compare the atleast one first reference level V₁ with a level generated from theanalog input signal V_(in) and the second reference levels, so as toperform the successive approximation procedure to obtain the posteriorcomparison result. It should be noted that the second levels V₂ areadjusted sequentially by adding the inputs of the second DAC 110 to theanalog input signal V_(in); and, the comparator circuit 115 is arrangedto compare the at least one first reference level V₁ with thesequentially adjusted level step-by-step to obtain the posteriorcomparison result. In this embodiment, the second levels V₂ are adjustedgradually each time when estimating one bit of the LSB portion by addingthe inputs of the second DAC 110 to the analog input signal V_(in).

The comparator circuit 115 is arranged to compare the correspondingresultant level with a selected first reference level V₁ provided by thefirst DAC 105, and a comparison result is outputted to the decoder 120so that the decoder 120 can determine a most important bit (the firstbit) of the LSB portion. Then, the decoder 120 is arranged toappropriately control on/off states of the transistor switches in thesecond DAC 110 so that the second DAC 110 provides a correspondingsecond level V₂ to the comparator circuit 115. Then, after thecomparison operation of the comparator circuit 115, the decoder 120 candetermine the second bit of the LSB portion. Thus, during the successiveapproximation procedure of this embodiment, by controlling on/off statesof the transistor switches in the second DAC 110 to step-by-step adjusta level of the analog input signal V_(in) based on the second referencelevels and comparing a corresponding first level V₁ with the adjustedlevels, the decoder 120 can correctly estimate the LSB portion accordingto the MSB portion estimated by the decoder 120 itself. The restoperation may be deduced by analogy, and is not detailed for brevity.

Specifically, the second DAC 110 is a capacitive DAC such as a capacitorarray, and the capacitive DAC 110 is used for receiving the secondreference levels which are used by the above-mentioned successiveapproximation procedure to estimate the LSB portion. In order tosignificantly decrease settling time of the capacitive DAC, the firstDAC 105 is used for estimate a portion of bit(s) (i.e. the MSB portion)of the digital output signal V_(out) before the successive approximationprocedure is employed for estimating the LSB portion. Accordingly, alevel range formed by the second reference levels can be greatlydecreased, and the settling time resulting from the successiveapproximation procedure can be greatly reduced. Thus, the whole timeperiod required for estimating the digital codeword of the digitaloutput signal V_(out) can also be greatly reduced. The level range ofthe second reference levels is decreased from a full swing range of theanalog input signal V_(in) to a portion of the full swing range ratherthan the full swing range. In this embodiment, the MSB portion includesMN bit(s), and the level range of the second reference levels isdetermined by dividing the full swing range by 2^(MN). The settling timeof the second DAC 110 is reduced due to the decreased level range. Also,the decreased level range is helpful for circuitry implementations inthe advance of MOS technology.

For achieving the purpose of decreasing settling time of the capacitiveDAC 110, in this embodiment, the first DAC 105 and the capacitive DAC110 share the same comparator circuit, i.e. the comparator circuit 115.That is, the operation of estimating the MSB portion shares the samecomparison operation of the comparator circuit 115 with the operation ofestimating the LSB portion. An estimation of the MSB portion and anestimation of the LSB portion are performed or obtained by using thesame comparison operation of the comparator circuit 115. Thus, it is notrequired to employ individual and over-range comparators for theoperation of estimating the MSB portion and error correction. As shownin FIG. 1, the comparator circuit 115 includes two input signal pathswherein one is arranged to receive the at least one first level V₁ andthe other is arranged to receive the second levels V₂. Detail operationis described in the following.

In this embodiment, the analog-to-digital conversion apparatus 100 isarranged to convert the analog input signal V_(in) into the digitaloutput signal V_(out) with a resolution of N bits. For example, N bitscan be eight bits; however, the bit resolution is merely used as anexample rather than a limitation of the present invention. The first DAC105 generates and provides first reference level(s) V₁ numbered 2^(MN)−1for the comparator circuit 115, and the comparator circuit 115 isarranged to estimate most significant bit(s) of the digital outputsignal V_(out). MN indicates that the MSB portion includes MN bit(s)wherein MN is a positive integer. In addition, LN indicates that the LSBportion includes LN bit(s) wherein LN is also a positive integer. Inthis embodiment, MN is equal to two, and LN is equal to six. That is,the first DAC 105 provides three first reference levels for thecomparator circuit 115, and the decoder 120 is used with the comparatorcircuit 115 to estimate two most significant bits (MSB) of the digitalcodeword of the digital output signal V_(out). These first referencelevels are implemented by employing ¼*V_(r), ½*V_(r), and ¾*V_(r) (i.e.,0.25V_(r), 0.5V_(r), and 0.75V_(r)) wherein V_(r) indicates a fullvoltage swing range of the analog input signal V_(in). The requiredreference voltage is lowered to ¾*V_(r) (i.e., 0.75V_(r)) for the analoginput signal V_(in) having a full swing V_(r).

In practice, the first DAC 105 may include three blocks 105A-105C thatare respectively used for generating the three different referencelevels (e.g., ¼*V_(r), ½*V_(r), and ¾*V_(r)). Each block of the firstDAC 105 is implemented by using a capacitor and a plurality of switchessuch as NMOS transistors. Each block of the first DAC 105 is notrequired to wait for a longer settling time of capacitorcharging/discharging due to switches' on/off switching because eachblock of the first DAC 105 in this embodiment is arranged to provide acorresponding and constant voltage reference level, respectively. Forinstance, the block 105A may be used for providing a reference level¾*V_(r), the block 105B may be used for providing a reference level½*V_(r), and the block 105C may be used for providing a reference level¼*V_(r). Also, the comparator circuit 115 may include three comparatorblocks 115A-115C corresponding to the above-mentioned blocks 105A-105Cof the first DAC 105. The three comparator blocks 115A-115C alsocorrespond to the three output nodes N1-N3 of the second DAC 110,respectively. The comparator blocks 115A-115C are arranged to comparethe level of the analog input signal V_(in) with the first referencelevels ¾*V_(r), ½*V_(r), and ¼*V_(r), respectively, to generate apreliminary comparison result and output the preliminary comparisonresult to the decoder 120. According to the preliminary comparisonresult, the decoder 120 can determine the MSB portion. When the MSBportion (two MSB bits) has been estimated, the decoder 120 buffers andrecords this MSB portion.

After estimating the MSB portion, the decoder 120 is used with thecomparator circuit 115 and the second DAC 110 to estimate the LSBportion based on the MSB portion and the successive approximationprocedure. When beginning execution of the successive approximationprocedure, the decoder 120 is arranged to control the on/off states ofswitches (e.g. NMOS transistors) in the second DAC 110 so as to providedifferent electric charges for the comparator blocks 115A-115C atdifferent times. By step-by-step controlling the on/off states of NMOStransistors in the second DAC 110 to exert different reference levels onthe analog input signal V_(in) at different times, the decoder 120gradually increases/charges the level of the analog input signal V_(in)to make the level approach to a first reference level corresponding tothe estimated MSB portion. After the level is enough close to the firstreference level corresponding to the estimated MSB portion, the decoder120 can determine the LSB portion based on the on/off states of theabove-described NMOS transistor switches. Thus, the digital codeword ofthe digital output signal V_(out) is determined since the MSB and LSBportions have been estimated.

It should be noted that, the second reference levels are used forestimating the LSB portion, and in an implementation the secondreference levels can be configured as a group of reference levelsnumbered 2^(LN) which are generated by uniformly dividing a level rangeof 0.25V_(r). For example, the second reference levels can be differentreference levels numbered 64. In another implementation, the secondreference levels (64 different levels) can be generated by a group oflevels that is numbered N smaller than 64. This can be achieved byincorporating capacitive interpolation into the second DAC 110. Anadvantage of fewer voltage levels to implement the second referencelevels is that the number of resistors in a resistor ladder used forgenerating the at least one first reference level and the secondreference levels can be reduced greatly. Please note that in thisembodiment, reference voltage levels inputted to the first DAC 105 andsecond DAC 110 are generated by using the above-mentioned resistorladder (not shown in FIG. 1); however, this is not intended to be alimitation of the present invention.

Further, the settling for the first reference levels (e.g., ¾*V_(r),½*V_(r), and ¼*V_(r)) is very fast since it is merely required to chargethe input capacitance of the comparator blocks 115A-115C, parasiticcapacitors, and other transistor switches. The settling for the secondlevels V₂ is also fast. For example, with reference to estimating themost important bit of the LSB portion in this embodiment is arranged tocause the second DAC 110 to exert half of ¼*V_(r) on the analog inputsignal Vin, rather than exerting half of V_(r) on the analog inputsignal Vin. Therefore, compared to a conventional successiveapproximation procedure, the successive approximation procedure of thepresent invention does not require waiting for a longer capacitorcharging settling time. That is, compared to the conventional successiveapproximation, the settling for the second levels V₂ in the embodimentbecomes very fast. The above-mentioned description is merely used as anexample for illustrative purposes but not intended to be a limitation ofthe present invention.

Please refer to FIG. 2, which depicts a diagram of a comparator blocksuch as 115A-115C as shown in FIG. 1. The comparator block includes acalibration block 1151, resistors R₁-R₂, a plurality of transistorsM₁-M₄ & S₁-S₂, and current sources I₁-I₂. The calibration block 1151 isarranged to generate and provide a calibrated differential signal V_(o)for the transistors M₁-M₄. When performing SAR procedure, the comparatorblock is used for receiving one of the first reference levels ¾*V_(r),½*V_(r), and ¼*V_(r) and a resultant signal generated by graduallyadding different second reference levels to the analog input signalV_(in). All signals or levels are configured to be implemented bydifferential signals. However, this is not meant to be a limitation ofthe present invention. As shown in FIG. 2, transistors M₁-M₄ are turnedoff during the sampling period for the analog input signal V_(in) toavoid a non-linear input capacitance due to unbalanced bias current. Foreach comparator block, a corresponding coarse reference level (e.g. oneof the first reference levels ¾*V_(r), ½*V_(r), and ¼*V_(r)) is appliedto gate terminals of the transistor switches M₁ and M₃ for estimatingthe MSB portion. During the period of the successive approximationprocedure, the resultant signal V₂ (i.e. a combination of the analoginput signal V_(in) with a corresponding second reference level) isapplied to gate terminal of the transistor switches M₂ and M₄ of thecomparator block. The common mode voltage of both the input differentialpairs is kept the same during the whole conversion period. This designensures that the maximum gain can be achieved.

In order to more clearly describe the operations of estimating the mostand least significant bit portions of this embodiment, FIG. 3A inconjunction with FIG. 3B respectively depict a diagram of the detailcircuitry elements of capacitors and comparator blocks 115A-115Caccording to another embodiment of the present invention and a diagramof the proposed ADC timing sequence according to this embodiment of thepresent invention. As shown in FIG. 3A in conjunction with FIG. 3B, eachcomparator block includes two differential input pairs. At Φ₅, theanalog input signal V_(in) and (V_(CM)−0.25V_(r)) are sampled incapacitors of 4C_(U) (C_(u) represents an unit capacitance). At thebeginning of first conversion cycle Φ₁, 0.5V_(r), 0.25V_(r) and zero areconnected to corresponding bottom plates (V_(rMSB1), V_(rMSB2), andV_(rMSB3)) of capacitors for comparison of the most significant bitportion. The settling of the first reference levels is very fast becausethe resistor ladder (not shown in FIG. 1) only needs to charge inputcapacitances of the comparator blocks 115A-115C and parasitic capacitorof 4C_(U) and six transistor switches. After the most significant bitportion is determined, V_(rMSB1)-V_(rMSB3) are decided and fixed in theconversion cycle of the least significant bit portion. At beginning ofsuccessive approximation procedure conversion Φ₂, V_(rLSB1) is switchedto 0.25V_(r) and V_(rLSB2) is kept at the ground level. The requiredsecond levels for the conversion of least significant bit portion areestablished by capacitive interpolation, and fast settling can beachieved due to reduced reference range of 0.25V_(r) and low turn-onresistance of NMOS transistor switches. In Φ₂-Φ₄, V_(rLSB1) switchesdownward to zero while V_(rLSB2) switches upward to 0.25V_(r). Themonotonic switching procedure costs less power than a conventionalsuccessive approximation ADC that is based on a trial-and-error searchprocedure.

Further, in order to prevent from using a high-frequency input clock,the proposed ADC timing sequence shown in FIG. 3B adopts an asynchronoustiming scheme, and corresponding control signals are generatedinternally. The analog input signal V_(in) is sampled at rising edge ofthe signal CK_(S) that is triggered by CK_(cmp) internally at the end ofprevious conversion period. The sampling instant is decided by risingedge of external clock CK_(ext) with arbitrary duty cycle.

In summary, in the embodiments of the present invention, by using thefirst DAC 105 with the comparator circuit 115, fast settling of thereference level (s) for the comparison of the most significant bitportion is obtained. Further, by using the capacitive interpolation, anumber of NMOS transistor switches can be reduced, and a matchingrequirement of corresponding circuitry elements can be lowered down. Inaddition, the need of specific duty cycle of an external clock fordefining sampling period in a conventional asynchronous successiveapproximation analog-to-digital converter can be avoided by using thetiming sequence proposed in FIG. 3B. In addition, according to theabove-mentioned design, the analog-to-digital conversion apparatus 100consumes less power and occupies a smaller area of a chip die. Also, alarge number of switches and a complicated routing are reduced greatly.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A method utilized in an analog-to-digitalconversion apparatus, for converting an analog input signal into adigital output signal, the digital output signal including a mostsignificant bit portion and a least significant bit portion, the methodcomprising: using a comparator circuit to compare the analog inputsignal with at least one first reference level to generate a preliminarycomparison result without using a successive approximation procedure,the at least one first reference level being used for determining themost significant bit portion; estimating the most significant bitportion according to the preliminary comparison result; based on thepreliminary comparison result, performing the successive approximationprocedure to obtain a posterior comparison result according to aplurality of second reference levels, the second reference levels beingused for determining the least significant bit portion; and estimatingthe least significant bit portion according to the posterior comparisonresult.
 2. The method of claim 1, further comprising: performing anestimation of the most significant bit portion and an estimation of theleast significant bit portion by using a same comparator circuit.
 3. Themethod of claim 2, wherein the step of performing the successiveapproximation procedure to obtain the posterior comparison resultcomprises: performing the successive approximation procedure to obtainthe posterior comparison result by using the comparator circuit tocompare the at least one first reference level with a level generatedfrom the analog input signal and the second reference levels.
 4. Themethod of claim 1, wherein a level range of the second reference levelsis a portion of a full swing range of the analog input signal ratherthan the full swing range.
 5. The method of claim 1, wherein the mostsignificant bit portion includes MN bit(s), and the level range of thesecond reference levels is determined by dividing the full swing rangeby 2^(MN), wherein MN is a positive integer.
 6. An analog-to-digitalconversion (ADC) apparatus for converting an analog input signal into adigital output signal, the digital output signal including a mostsignificant bit portion and a least significant bit portion, theanalog-to-digital conversion apparatus comprising: a firstdigital-to-analog converter (DAC), for providing at least one firstreference level for a comparator circuit, the at least one firstreference level being used for determining the most significant bitportion; a second DAC, for providing a plurality of second referencelevels for the comparator circuit, the second reference levels beingused for determining the least significant bit portion; the comparatorcircuit, coupled to the first DAC and the second DAC, for comparing theanalog input signal with the at least one first reference level togenerate a preliminary comparison result without using a successiveapproximation procedure; a decoder, coupled to the comparator circuit,for estimating the most significant bit portion according to thepreliminary comparison result; wherein based on the preliminarycomparison result, the decoder is used with the comparator circuit toperform the successive approximation procedure to obtain a posteriorcomparison result according to the second reference levels, and thedecoder is arranged to estimate the least significant bit portionaccording to the posterior comparison result.
 7. The ADC apparatus ofclaim 6, wherein the comparator circuit is arranged to compare the atleast one first reference level with a level generated from the analoginput signal and the second reference levels, so as to perform thesuccessive approximation procedure to obtain the posterior comparisonresult.
 8. The ADC apparatus of claim 6, wherein a level range of thesecond reference levels is a portion of a full swing range of the analoginput signal rather than the full swing range.
 9. The ADC apparatus ofclaim 6, wherein the most significant bit portion includes MN bit(s),and the level range of the second reference levels is determined bydividing the full swing range by 2^(MN), wherein MN is a positiveinteger.
 10. An analog-to-digital conversion (ADC) apparatus forconverting an analog input signal into a digital output signal, thedigital output signal including a first portion and a second portion,the analog-to-digital conversion apparatus comprising: a firstdigital-to-analog converter (DAC), for providing at least one firstreference level being used for determining the first portion; a secondDAC, for providing at least one second reference level being used fordetermining the second portion, and for sampling and storing the analoginput signal; a comparator circuit, coupled to the first DAC and thesecond DAC, for comparing the analog input signal with the at least onefirst reference level to generate a preliminary comparison result at anoutput of the comparator circuit, and generating a posterior comparisonresult at the output according to the analog input signal and the atleast one second reference level; and a decoder, coupled to thecomparator circuit, for estimating the first portion according to thepreliminary comparison result, and estimating the second portionaccording to the posterior comparison result.
 11. The ADC apparatus ofclaim 10, wherein the comparator circuit further comprises a first inputcoupled to the first DAC and a second input coupled to the second DAC.